149 research outputs found
Bounds on Superconformal Theories with Global Symmetries
Recently, the conformal-bootstrap has been successfully used to obtain
generic bounds on the spectrum and OPE coefficients of unitary conformal field
theories. In practice, these bounds are obtained by assuming the existence of a
scalar operator in the theory and analyzing the crossing-symmetry constraints
of its 4-point function. In superconformal theories with a
global symmetry there is always a scalar primary operator, which is the top of
the current-multiplet. In this paper we analyze the crossing-symmetry
constraints of the 4-point function of this operator for
theories with global symmetry. We analyze the current-current OPE, and
derive the superconformal blocks, generalizing the work of Fortin,
Intrilligator and Stergiou to the non-Abelian case and finding new
superconformal blocks which appear in the Abelian case. We then use these
results to obtain bounds on the coefficient of the current 2-point function.Comment: Corrected error in analysis for U(1) symmetr
Gate Defined Quantum Confinement in Suspended Bilayer Graphene
Quantum confined devices that manipulate single electrons in graphene are
emerging as attractive candidates for nanoelectronics applications. Previous
experiments have employed etched graphene nanostructures, but edge and
substrate disorder severely limit device functionality. Here we present a
technique that builds quantum confined structures in suspended bilayer graphene
with tunnel barriers defined by external electric fields that break layer
inversion symmetry, thereby eliminating both edge and substrate disorder. We
report clean quantum dot formation in two regimes: at zero magnetic field B
using the single particle energy gap induced by a perpendicular electric field
and at B > 0 using the quantum Hall ferromagnet {\nu} = 0 gap for confinement.
Coulomb blockade oscillations exhibit periodicity consistent with electrostatic
simulations based on local top gate geometry, a direct demonstration of local
control over the band structure of graphene. This technology integrates single
electron transport with high device quality and access to vibrational modes,
enabling broad applications from electromechanical sensors to quantum bits.Comment: 22 pages, 9 figures, includes supplementary informatio
Local Spin Susceptibilities of Low-Dimensional Electron Systems
We investigate, assess, and suggest possibilities for a measurement of the
local spin susceptibility of a conducting low-dimensional electron system. The
basic setup of the experiment we envisage is a source-probe one. Locally
induced spin density (e.g. by a magnetized atomic force microscope tip) extends
in the medium according to its spin susceptibility. The induced magnetization
can be detected as a dipolar magnetic field, for instance, by an
ultra-sensitive nitrogen-vacancy center based detector, from which the spatial
structure of the spin susceptibility can be deduced. We find that
one-dimensional systems, such as semiconducting nanowires or carbon nanotubes,
are expected to yield a measurable signal. The signal in a two-dimensional
electron gas is weaker, though materials with high enough -factor (such as
InGaAs) seem promising for successful measurements.Comment: 11 pages, 12 figure
Enhancing the Coherence of a Spin Qubit by Operating it as a Feedback Loop That Controls its Nuclear Spin Bath
In many realizations of electron spin qubits the dominant source of
decoherence is the fluctuating nuclear spin bath of the host material. The
slowness of this bath lends itself to a promising mitigation strategy where the
nuclear spin bath is prepared in a narrowed state with suppressed fluctuations.
Here, this approach is realized for a two-electron spin qubit in a GaAs double
quantum dot and a nearly ten-fold increase in the inhomogeneous dephasing time
is demonstrated. Between subsequent measurements, the bath is prepared
by using the qubit as a feedback loop that first measures its nuclear
environment by coherent precession, and then polarizes it depending on the
final state. This procedure results in a stable fixed point at a nonzero
polarization gradient between the two dots, which enables fast universal qubit
control.Comment: Journal version. Improved clarity of presentation and more concise
terminology. 4 pages, 3 figures. Supplementary document included as ancillary
fil
Tunneling Spectroscopy of Disordered Two-Dimensional Electron Gas in the Quantum Hall Regime
Recently, Dial et al. presented measurements of the tunneling density of
states into the bulk of a two dimensional electron gas under strong magnetic
fields. Several high energy features appear in the measured spectrum showing a
distinct dependence on filling factor and a unique response to temperature. We
present a quantitative account of the observed structure, and argue it results
from the repulsive Coulomb interactions between the tunneling electron and
states localized at disorder potential wells. The quenching of the kinetic
energy by the applied magnetic field leads to an electron addition spectrum
that is primarily determined by the external magnetic field and is nearly
independent of the disorder potential. Using a Hartree-Fock model we reproduce
the salient features of the observed structure
Tuning Topological Superconductivity in Phase-Controlled Josephson Junctions with Rashba and Dresselhaus Spin-Orbit Coupling
Recently, topological superconductors based on Josephson junctions in
two-dimensional electron gases with strong Rashba spin-orbit coupling have been
proposed as attractive alternatives to wire-based setups. Here, we elucidate
how phase-controlled Josephson junctions based on quantum wells with [001]
growth direction and an arbitrary combination of Rashba and Dresselhaus
spin-orbit coupling can also host Majorana bound states for a wide range of
parameters as long as the magnetic field is oriented appropriately. Hence,
Majorana bound states based on Josephson junctions can appear in a wide class
of two-dimensional electron gases. We study the effect of spin-orbit coupling,
the Zeeman energies, and the superconducting phase difference to create a full
topological phase diagram and find the optimal stability region to observe
Majorana bound states in narrow junctions. Surprisingly, for equal Rashba and
Dresselhaus spin-orbit coupling, well localized Majorana bound states can
appear only for phase differences as the topological gap
protecting the Majorana bound states vanishes at . Our results show
that the ratio between Rashba and Dresselhaus spin-orbit coupling or the choice
of the in-plane crystallographic axis along which the superconducting phase
bias is applied offer additional tunable knobs to test Majorana bound states in
these systems. Finally, we discuss signatures of Majorana bound states that
could be probed experimentally by tunneling conductance measurements at the
edge of the junction.Comment: 21 pages, 12 figure
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